Interference filter with sharp cutoff characteristics

ABSTRACT

An interference filter for use in a copying machine comprises a glass substrate and a plurality of high-refractive index and low-refractive index layers alternately superposed on the substrate for reducing ripples at opposite sides of the reflective range. The first and second layers counted from the substrate have a greater optical layer thickness than λo/2. The third and fourth layers have a greater optical layer thickness than λo/4. The second top layer has a greater optical layer thickness than λo/2. The top layer has a smaller optical layer thickness than λo/4. The remaining layers have an optical layer thickness equal to λo/4.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color separation filter for use in acolor copier and, more particularly, a filter to improve sidebandcharacteristics.

2. Description of the Prior Art

In an electrostatic copying system, a document placed on a transparentplate is illuminated by an appropriate illuminating device, and adocument image is projected through a lens mounted on a copying opticalpath onto a uniformly charged photoconductive member for forming anelectrostatic latent image thereon. This latent image is developed byapplying thereto a powder toner charged to have opposite characteristicsto the photoconductive member. The resulting toner image is thereaftertransferred and fixed to copying paper, OHP paper, a postcard or thelike. In recent years, vigorous developments have been made in the artof producing color copies with a color separation filter mounted on theoptical path and a developing device having a plurality of developersincluding color toners corresponding to the color separation filter. Thecolor separation filter plays a technically important part in such acolor copying system, and is required to have high performance.

Generally, the color separation filter comprises an absorption filterwhich absorbs pigments, or an interference filter which interferes withlight. The latter is often preferred in designing the color separationfilter because of the greater freedom of designing and the higherefficiency of color separation.

In designing the interference filter, the following three conditions arefirst determined:

(1) the wavelength of the reflective range,

(2) the width of the reflective range, and

(3) the residual transmittance of the reflective range.

Assume here that the reflective range has a center wavelength Tλo, awidth Δλo and a residual transmittance Tλo. The reflective rangecentering on wavelength λo is generated where, as shown in FIG. 3,dielectric elements having a high refractive index nH and those having alow refractive index nL are stacked each in an optical thickness λo/4 ona substrate G. At this time, the width Δλo of the reflective range isdetermined by a ratio between the high refractive index nH and lowrefractive index nL. Conversely speaking, the width Δλo of thereflective range is controllable by the ratio between the highrefractive index nH and low refractive index nL.

Further, the residual transmittance Tλo of the reflective range iscontrollable by the number of stacked layers. That is, the residualtransmittance Tλo decreases with an increase in the number of layers.

This is illustrated in FIGS. 4 through 7.

The number of layers increases progressively from FIG. 4 to FIG. 7. Inthis state, the residual transmittance Tλo of the reflective rangedecreases progressively from FIG. 4 to FIG. 7, with progressively largerripples occurring at opposite sides of the reflective range. Theseripples reduce the transmittance, to the detriment of the colorseparation filter performance.

FIG. 8 shows a conventional layer arrangement for reducing theseripples. This multi-layer interference filter includes, counted from thesubstrate G, a first layer S1 of a high-refraction dielectric element, asecond layer S2 of a low-refraction dielectric element, a third layer S3of the high-refraction dielectric element, and so on up to an Nth layerSN of the low refraction dielectric element. The first layer S1 and thelast layer SN have an optical thickness λo/8, respectively, while eachof the other layers has an optical thickness λo/4. This construction iseffective for diminishing the ripples at the opposite ends of thereflective range as shown in FIG. 9.

However, the ripples do remain even with this construction, to such adegree to impair highly efficient color separation.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the interference filterof the type noted above for reducing the ripples occurring in oppositetransmission ranges of greater and smaller wavelengths across thenon-transmission range, to enable highly efficient color separation.

The above object is achieved, according to the present invention, by aninterference filter comprising a substrate for carrying a plurality, N,of layers superposed thereon, the layers including layers of ahigh-refractive index element arranged at odd-numbered places in orderas counted from the substrate, and layers of a low-refractive indexelement arranged at even-numbered places in order as counted from thesubstrate, wherein the first layer counted from the substrate has agreater optical layer thickness than λo/2, the second layer counted fromthe substrate has a greater optical layer thickness than λo/2, the thirdlayer counted from the substrate has a greater optical layer thicknessthan λo/4, the fourth layer counted from the substrate has a greateroptical layer thickness than λo/4, the N-1th layer counted from thesubstrate has a greater optical layer thickness than λo/2, and the Nthlayer counted from the substrate has a smaller optical layer thicknessthan λo/4.

The interference filter as constructed above is capable of drasticallyreducing the ripples at opposite sides of the reflective range, asevident from its spectral transmittance illustrated in FIG. 2.

The interference filter according to the present invention enableshighly efficient color separation, now that only greatly diminishedripples occur in opposite transmission ranges of greater and smallerwavelengths across the non-transmission range or side bandwidth range.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show an interference filter according to the presentinvention, in which:

FIG. 1 is an enlarged view of the interference filter of the presentinvention, and

FIG. 2 is a graph showing spectral transmittance of the interferencefilter of FIG. 1.

FIGS. 3 through 9 show the basic concept of an interference filter andconventional interference filters, in which:

FIG. 3 is and enlarged view of a conventional interference filter,

FIGS. 4 through 7 are graphs showing variations in the spectraltransmittance occurring with the interference filter of FIG. 3,

FIG. 8 is a view of a conventional interference filter having adifferent layer construction for improved ripple characteristics, and

FIG. 9 is a graph showing spectral transmittance of the interferencefilter of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described hereinafterwith reference to the drawings. FIG. 1 is a schematic enlarged viewshowing the construction of an interference filter according to theinvention. This filter has a multi-layer construction including, countedfrom a glass substrate G, a first layer S1 of a high-refractive indexdielectric element H, a second layer S2 of a low-refractive indexdielectric element L, a third layer S3 of the high-refractive indexdielectric element H, and so on up to an Nth layer SN of the lowrefractive index dielectric element L. The number of layers N in thisembodiment is 20 to 40, preferably 30, to produce advantageous effects.Layer thicknesses of the high-refractive index and low refractive indexdielectric elements H and L represent optical thickness. Thehigh-refractive dielectric element may comprise titanium dioxide (TiO₂),zirconium dioxide (ZrO₂) or zinc sulfide (ZnS). The low-refractive indexdielectric element may comprise silicon dioxide (SiO₂), magnesium oraluminum oxide (Al₂ O₃).

The glass substrate has a refractive index of about 1.52.

The following table shows a specific arrangement according to thisembodiment.

    ______________________________________                                                            refractive  thickness                                     layer     Substance index       s                                             ______________________________________                                        Substrate Glass     1.52                                                      S1        ZrO.sub.2 2.05        2.96 × λo/4                      S2        AL.sub.2 O.sub.3                                                                        1.63        2.96 × λo/4                      S3        ZrO.sub.2 2.05        1.06 × λo/4                      S4        AL.sub.2 O.sub.3                                                                        1.63        1.06 × λo/4                      S5        ZrO.sub.2 2.05        λo/4                                   S6        AL.sub.2 O.sub.3                                                                        1.63        λo/4                                   SN-3      ZrO.sub.2 2.05        λo/4                                   SN-2      AL.sub.2 O.sub.3                                                                        1.63        λo/4                                   SN-1      ZrO.sub.2 2.05        2.24 × λo/4                      SN        AL.sub.2 O.sub.3                                                                        1.63        0.84 × λo/4                      ______________________________________                                    

According to the present invention, the first layer S1 of thehigh-refractive index element H having a greater layer thickness thanλo/2 is placed on the glass substrate G, and then the second layer S2 ofthe low-refractive index element L again having a greater layerthickness than λo/2 is placed on the first layer S1. Then, the thirdlayer of the high refractive index element H and the fourth layer of thelow-refractive index element L, both having a greater thickness thanλo/4 are placed on top of the layers S1 and S2. The fifth to N-2thlayers having a thickness λo/4 are successively overlaid with thehigh-refractive index element H and low-refractive index element Larranged alternately. The uppermost layer SN of the low-refractive indexelement L has a thickness less than λo/4, with the second highest layerSN-1 of the high-refractive index element H having a greater thicknessthan λo/2.

FIG. 2 is a graph showing spectral transmittance of the interferencefilter as constructed above. As seen from this graph, the ripples at theopposite sides of the absorption band range are significantly reduced ascompared with the ripples shown in FIG. 9.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the present invention, they should be construedas being included therein.

What is claimed is:
 1. An interference filter for use in a copyingmachine to be positioned in an optical path of light passing through animage-forming lens for preventing transmission of light having apredetermined wavelength range about a wavelength, λo, said interferencefilter comprising:a substrate for carrying a plurality, N, of layerssuperimposed thereon; said layers including layers of a high-refractiveindex element arranged at odd-numbered places in order as counted fromsaid substrate, layers of a low-refractive index element arranged ateven-numbered places in order as counted from said substrate, and theNth layer as counted from said substrate being of a low-refractive indexelement; wherein: the first layer as counted from said substrate has anoptical layer thickness greater than λo/2, the second layer as countedfrom said substrate has an optical layer thickness greater than λo/2,the third layer as counted from said substrate has an optical layerthickness greater than λo/4, the fourth layer as counted from saidsubstrate has an optical layer thickness greater than λo/4, the N-1layer as counted from said substrate has an optical layer thicknessgreater than λo/2, and the Nth layer as counted from said substrate hasan optical layer thickness smaller than λo/4.
 2. An interference filteras claimed in claim 1, wherein the fifth to N-2 layers counted from saidsubstrate have an optical layer thickness equal to λo/4.
 3. Aninterference filter as claimed in claim 1, wherein the first layer has agreater optical layer thickness than the N-1 layer.
 4. An interferencefilter comprising:a substrate for carrying a plurality, N, of layerssuperimposed thereon; said layers including layers of a high-refractiveindex element arranged at odd-numbered places in order as counted fromsaid substrate, layers of a low-refractive index element arranged ateven-numbered places in order as counted from said substrate, and theNth layer as counted from said substrate being of a low-refractive indexelement; wherein: the first layer and a second layer as counted fromsaid substrate have substantially the same optical layer thickness, thethird layer and a fourth layer as counted from said substrate havesubstantially the same optical layer thickness, the N-1 layer as countedfrom said substrate has an optical layer thickness smaller than saidfirst layer, the Nth layer as counted from said substrate has an opticallayer thickness smaller than said fourth layer, and the remaining layershave substantially the same optical layer thickness which is between theoptical layer thicknesses of said fourth layer and said Nth layer.
 5. Aninterference filter as claimed in claim 4, wherein;the optical layerthickness of the first and second layers is 2.96*λo/4, the optical layerthickness of the third and fourth layers is 1.06*λo/4, the optical layerthickness of the N-1 layer is 2.24*λo/4, the optical layer thickness ofthe Nth layer is 0.84*λo/4, and the optical layer thickness of theremaining layers is λo/4.
 6. An interference filter as claimed in claim5, wherein said substrate is formed of glass, said high-refractive indexlayers are formed of zirconium dioxide, and said low-refractive indexlayers are formed of aluminum oxide.
 7. An interference filter asclaimed in claim 5, wherein said substrate has a refractive index of1.52, said high-refractive index layers have a refractive index of 2.05,and said low-refractive index layers have a refractive index of 1.63. 8.An interference filter as claimed in claim 5, wherein the number oflayers superimposed on said substrate is 20 to 40, preferably
 30. 9. Aninterference filter for use in a copying machine as mounted on anoptical path of light passing through an image-forming lens forpreventing transmission of light having a predetermined wavelength rangeabout a design wavelength, λo, said interference filter comprising:asubstrate for carrying a plurality, N, of layers superimposed thereon;said layers including layers of high-refractive index elements arrangedat odd-numbered places in order as counted from said substrate, andlayers of low-refractive index elements arranged at even-numbered placesin order as counted from said substrate; wherein: the first layer ascounted from said substrate has an optical layer thickness greater thanλo/2, the second layer as counted from said substrate has an opticallayer thickness greater than λo/2, the third layer as counted from saidsubstrate has an optical layer thickness greater than λo/4, the fourthlayer as counted from said substrate has an optical layer thicknessgreater than λo/4, the fifth to N-2 layers as counted from saidsubstrate have an optical layer thickness equal to λo/4, the N-1 layeras counted from said substrate has an optical layer thickness greaterthan λo/2, and the Nth layer as counted from said substrate has anoptical layer thickness smaller than λo/4.